Compressed elastomer process for autofrettage and lining tubes

ABSTRACT

An inventive process is provided for creating residual compressive stress at a surface of a structure without resort to custom mandrels and dangerous high-pressure fluids. The inventive process yields autofrettage of a structure such as a tube, gun barrel and the like, the structure having an outer surface and an inner surface.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application Ser.No. 60/955,441 filed Aug. 13, 2007, the entire contents of which areincorporated herein by reference.

GOVERNMENT INTEREST

The invention described herein may be manufactured, used, and licensedby or for the United States Government.

FIELD OF THE INVENTION

The present invention relates in general to a process for autofrettaginga structure, and in particular, to a process for autofrettaging astructure, for example as a gun barrel, using an expander.

BACKGROUND OF THE INVENTION

It is well known that a residual compressive stress at a componentsurface can increase the strength and fatigue life of the component. Forexample, residual compressive stresses imposed on inner surfaces oftubes can provide resistance to fatigue and inhibit crack initiation andthe rate of crack propagation. Such results are possible since thepropagation of a crack requires tensile stresses to be present at thetip of the crack, and if a surface is under compression, the compressivestress must be overcome—in addition to any tensile stresses required forcrack initiation and/or crack propagation—before the crack can becreated or propagate if already present.

One process to produce residual compressive stress to a surface iscalled autofrettage. Autofrettage is a process wherein pressure isapplied within a containers e.g. a tube, such that the outer surface ofthe container undergoes elastic deformation whereas the inner surfaceundergoes elastic plus plastic deformation. After the pressure isremoved, the outer surface recovers the elastic strain but the innersurface recovers only the elastic strain with the plastic strainresulting in a residual compressive stress being present.

Autofrettage plastic deformation to the interior of a tube can becreated in a number of ways, including the use of explosives, hydraulicpressure, or mechanical force. For example, mechanical autofrettage usesa press to force an oversized mandrel through a tube, thereby causingthe inner surface of the tube to yield in tension while the material atthe outer surface of the tube remains elastic. After the mandrel haspassed through the tube, relaxation of the material results in adistribution of residual stress that is compressive on the innersurface.

In addition to mechanical autofrettage, hydraulic autofrettage can beaccomplished by placing a fluid within a sealed container and applyingpressure. Looking at FIGS. 1-3, a pump 20 can apply pressure to ahydraulic liquid 40 within a tube 10. The tube 10 has an outer surface12 and an inner surface 14. Also included are high-pressure seals 30which prevent the liquid 40 under high pressure from escaping the tube10. An internal pressure p_(i) within the interior of the tube 10 cancause elastic strain ε_(od) on the outer surface 12 and elastic plusplastic strain ε_(id) on the inner surface 14 as illustrated in FIG. 2.After the pressure is removed, the elastic strain is recovered at theouter surface 12 and inner surface 14, however the plastic strain at theinner surface 14 results in the presence of a residual compressivestress σ_(id). In this manner, a tube 10 having residual compressivestress at the inner surface 14 is provided.

Although such processes have been used to produce autofrettage withintubes such as gun barrels, processing equipment, high-pressure pumpcylinders, and the like, the mechanical autofrettage process requiresthe manufacture of mandrels that are specially designed and dimensionedfor the particular tube, gun barrel, etc. In addition, the use ofhydraulic autofrettage requires the use of high-pressure seals to beattached to the tube, gun barrel, etc., and with equipment failure canresult in the rapid and/or uncontrolled release of high-pressure fluid.Such a release is dangerous and requires additional safety equipment tobe used with hydraulic autofrettage systems. Therefore, an improvedprocess that provides for autofrettage would be desirable.

SUMMARY OF THE INVENTION

An inventive process is provided for creating residual compressivestress at a surface of a structure without resort to custom mandrels anddangerous high-pressure fluids. The inventive process yieldsautofrettage of a structure such as a tube, gun barrel and so forth, thestructure having an outer surface and an inner surface. Exemplarystructures that may be autofrettaged using processes of the presentinvention include, but are not limited to: gun barrels, pipes, tubes andother tubular articles for example a ¾ pipe, tanks, pressure vessels,containers and so forth. In addition, the process provides an expanderthat is placed adjacent the inner surface of the structure and acompressive force is applied thereto. In desirable embodiments, thecompressive force is an axial force. The compressive force along theaxial direction of the expander causes it to expand in a radialdirection as it is compressed in the axial direction. The expansion ofthe expander in the radial direction results in pressure being exertedon the interior surface of the structure and autofrettage thereof. Insome instances, the expander is made from an elastomer. In addition, theamount of autofrettage induced on the inner surface along a length ofthe structure can be varied and thereby provide a graded autofrettagedstructure.

The inventive process can also include providing a liner intermediatebetween the expander and the structure. Thereafter, with application ofaxial compressive force to the expander, an outer surface of the lineris bonded to the inner surface of the structure and/or inducement ofautofrettage of the liner is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an internal pressure within astructure;

FIG. 2 is a schematic illustration of strain and stress present at anouter surface and an inner surface of the structure shown in FIG. 1;

FIG. 3 is a schematic illustration of a prior art process for theautofrettaging of the structure shown in FIG. 1;

FIG. 4 is a perspective view of an embodiment of the present invention;

FIG. 5 is a side cross-sectional view of the embodiment shown in FIG. 4;and

FIG. 6 is a side cross-sectional view of a different embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention discloses an inventive process for autofrettagingstructures. As such, the process has utility for providing suchstructures with increased fatigue resistance.

The inventive process includes providing a structure that has an outersurface and an inner surface. An expander is placed adjacent to theinner surface of the structure and an axial compressive force is appliedthereto. Preferably, the expander is made from an elastomer or a plastichaving a Poisson's ratio between 0.3 and 0.5 inclusive and, morepreferably, an elastomer or a plastic having a Poisson's ratio between0.4 and 0.5 inclusive. The expander can be a solid form such as a plug,a particulate such as granules or shredded material, or a combination ofboth.

With the compressive force applied along the axial direction of theexpander, it expands in a radial direction and thereby provides apressure to the inner surface of the structure. It is appreciated thatthe structure can be a hollow cylindrically shaped structure such as atube, gun barrel, high-pressure pump cylinder, and the like.

The axial compressive force applied to the expander is of sufficientmagnitude to cause radial expansion of the expander and result inpredominantly elastic strain to the outer surface and elastic plusplastic strain to the inner surface of the structure. It is appreciatedthat the elastic and plastic strains can be radial and/or radial plusaxial in nature. In this manner, when the axial compressive force isremoved from the expander, the elastic strain is recovered at the outersurface of the structure, however the plastic strain at the innersurface results in a compressive residual stress thereon. Preferablyaxially compressive force is applied to one end of the expander, howeverthis is not required. In addition, it is appreciated that an expandercan take a number of shapes, illustratively including shapes for use inautofrettaging cylindrically-shaped components, conically-shapedcomponents, spherically-shaped components and the like.

Optionally, a liner is placed intermediate between the expander and theinner surface of the structure. Thereafter, axial compressive force isapplied to the expander, thereby resulting in an outwardly directedpressure being applied to the liner. This outward pressure can result inthe bonding of an outer surface of the liner to the inner surface of thestructure and/or plastic deformation to the liner. For the purposes ofthe present invention, the term bonding includes frictional bondingbetween two surfaces, mechanical interlocking between two surfaces andcombinations thereof. In this manner, bonding of the liner to thestructure and/or autofrettage of the liner are accomplished.

Turning now to FIGS. 4 and 5, an embodiment of the process disclosedherein is illustrated. As shown in FIG. 4, a structure in the form of atube 10 having an outer surface 12 and an inner surface 14 is provided.Within the tube 10 is an expander 70 with end caps 60 at opposite axialends of the expander 70. As illustrated by the arrows in FIG. 4, anaxial compressive force is applied to the end caps 60 and therebytransmitted to the expander 70. In certain preferred embodiments, theexpander 70 is be made from an elastomer and/or a plastic so that theexpander 70 has a Poisson's ratio between about 0.3 and about 0.5,inclusive. In certain more preferred embodiments, the expander 70 is bemade from an elastomer and/or a plastic so that the expander 70 has aPoisson's ratio between about 0.4 and about 0.5, inclusive.

Looking specifically at FIG. 5, as the axial compressive force isapplied to the expander 70, a radial force is applied to the innersurface 14 of the tube 10 as illustrated by the horizontal arrows. Theradial force predominantly induces elastic strain at the outer surface12, whereas elastic plus plastic strain is produced at the inner surface14. Thereafter, the axial compressive force is removed, along with theend caps 60 and the expander 70, with the tube 10 having a residualcompressive stress at the inner surface 14 having been produced.

The length of the tube 10 can be varied and components such as gunbarrels, processing tubing equipment, high-pressure pump cylinders andthe like, or in the alternative, conically-shaped components,hemispherically-shaped components, spherically-shaped components, andthe like can be autofrettaged using the inventive process disclosedherein.

In some instances, the end caps 60 can also undergo radial expansion andthereby not allow extrusion of the expander 70 between the end caps 60and the inner surface 14 of the tube 10. In this manner, the tolerancebetween an outer diameter of the end caps 60 and an inner diameter ofthe tube 10 does not have to be as stringent as with previous processesof autofrettaging. As such, it is appreciated that the use of theexpander 70 results in high-pressure seals not being required.

Turning now to FIG. 6, where like numerals correspond to those used withrespect to the aforementioned figures, the tube 10 has a liner 80 placedtherein. The liner 80 has an outer surface 82 and an inner surface 84.Within the liner 80, is placed an expander 90. As illustrated by thebroad vertical arrows in FIG. 6, an axial compressive force is appliedto the expander 90 through the use of hydraulic cylinders 92 and endcaps 60.

The horizontal arrows in FIG. 6 illustrate that the application of theaxial compressive force results in radial expansion of the expander 90.The radial expansion provides an internal pressure applied to the liner80. In some instances, the internal pressure is sufficient to bond theouter surface 82 of the liner 80 to the inner surface 14 of the tube 10or produce autofrettage of the liner 80. Optionally, pressures areapplied to simultaneously bond the liner 80 to the tube 10 and produceautofrettage of the liner 80. In addition to bonding of the liner 80 tothe tube 10, plastic strain can be produced at least partially withinthe wall thickness of the liner 80 and thereby result in residualcompressive stresses being present within the liner 80 after the axialcompressive force has been removed. In this manner, the liner 80 isbonded to the tube 10 and the liner is optimally autofrettaged in oneprocessing step.

The structure and the liner are made from any material known to thoseskilled in the art illustratively including metals and alloys. In someinstances, the structure is made from a steel, stainless steel,nickel-based alloy, cobalt-based alloy, and the like. In addition, theliner can be made from low strain-to-failure materials such asintermetallics, niobium, tungsten, molybdenum, alloys thereof and thelike. In this manner, a liner having relatively high hardness can beplaced and bonded within a structure such as a gun barrel. The expandercan be made from any material known to those skilled in the art and ispreferably made from a material that has a Poisson's ratio is between0.3 and 0.5, inclusive. For example, rubber has a Poisson's ratiogenerally equal to 0.5 and can be used as the expander material.

It is appreciated that the process can be used to apply differentmagnitudes of residual compressive stress as a function of locationwithin a structure. For example, using an expander of significant lesslength than a tube, the expander can be placed at different locationsalong the length of the tube with the magnitude of axial compressiveforce applied to the expander being a function the expander location. Inthe alternative, an expander having a property such as an outerdiameter, physical form (e.g. solid form versus particulate) and/orcomposition that varies as a function of an axial length of theexpander, can be placed within the tube and axial compressive forceapplied thereon. Thus a different internal pressure is applied to theinner surface of the tube, or a liner placed within the tube, as afunction of location. In this manner, a gradient of residual compressivestresses can be imposed along the length of the tube.

Due to friction between the expander and the tube or liner, there is apressure gradient along the length of the tube or liner when a load isapplied to the expander. The highest pressure will be near the pointwhere the load is applied, and the lowest pressure will be at a pointfarthest from where the load is applied. This friction can be reduced bythe use of a lubricant between the expander and the inner surface of thetube. The lubricant can be any material known by those skilled in theart to reduce the friction between the surfaces. Suggested lubricantsinclude, but are not limited to, erucic acid, graphite, oil, and soforth. If a lubricant is not used, the pressure gradient will result ina variation in autofrettage, with the greater degree of autofrettageoccurring where the pressure is the highest. The bond strength betweenthe liner and tube will also vary with the pressure. Thus in certaindesirable embodiments, a method of the present invention furtherincludes providing a lubricant between the expander and the innersurface of the structure.

The state of residual stress will vary along the axial length of thetubes since the pressure is only generated where the expander is incontact with the tube wall. The ends of the tube that have no contactwith the expander will have little to no bond between the structure andliner, and will not possess a state of autofrettage. If it is desired tohave the length of the tube to possess a bond or a state ofautofrettage, the tubes can be made with excess length that can beremoved. An alternative approach includes making a liner with excessivelength and further including an auxiliary structure that supports theends of the liner. After the process is completed, the auxiliarystructure is removed, and the ends of the liner are cut off.

The foregoing description is illustrative of particular embodiment ofthe invention, but is not meant to be a limitation upon the practicethereof. The following claims, including all equivalents thereof areintended to define the scope of the invention.

We claim:
 1. A process for autofrettaging wherein the process comprisesimposing plastic deformation on an inner surface of a metal structureand elastic deformation on a more outer portion of the structure, theprocess comprising: providing a metal structure having an outer surfaceand an inner surface; providing an expander that comprises an elastomeror a plastic; placing a liner intermediate between the expander and themetal structure having an outer surface and an inner surface, said linerhaving an outer surface and an inner surface; placing the expanderadjacent the inner surface of the liner; and applying a compressiveforce onto the expander wherein the elastomeric or plastic expanderoccupies a majority of the volume of the metal structure and has aPoisson's ratio between 0.4 and 0.5.
 2. The process of claim 1, whereinthe metal structure having an outer surface and an inner surface is ahollow cylindrically shaped structure.
 3. The process of claim 2,wherein the hollow cylindrically shaped structure is made from a madefrom a steel alloy, a stainless steel alloy, a nickel-based alloy or acobalt-based alloy.
 4. The process of claim 3, wherein the hollowcylindrically shaped structure is a gun barrel.
 5. The process of claim1, wherein the compressive force is an axial force.
 6. A structureproduced by the process of claim
 1. 7. The process of claim 1, whereinthe expander has a form selected from the group consisting of a solidform, a particulate and combinations thereof.
 8. A process forautofrettaging wherein the process comprises imposing plasticdeformation on an inner surface of a structure and elastic deformationon a more outer portion of the structure, the process comprising:providing a structure having an outer surface and an inner surface;providing an elastomeric or plastic expander that occupies a majority ofthe interior volume of the structure; placing the expander adjacent theinner surface of the structure; and applying a compressive force ontothe expander wherein the compressive force induces predominantly elasticstrain to the outer surface of the structure and elastic plus plasticstrain to the inner surface of the structure further wherein thecompressive force induces predominantly elastic strain to the outersurface of the structure and elastic plus plastic strain to the innersurface of the structure.
 9. A structure produced by the process ofclaim
 8. 10. A process for autofrettaging wherein the process comprisesimposing plastic deformation on an inner surface of a structure andelastic deformation on a more outer portion of the structure, theprocess comprising: providing a structure having an outer surface and aninner surface; providing an elastomeric or plastic expander thatoccupies a majority of the interior volume of the structure; placing theexpander adjacent the inner surface of the structure; and applying acompressive force onto the expander wherein the compressive forceinduces predominantly elastic strain to the outer surface of thestructure and elastic plus plastic strain to the inner surface of thestructure further wherein the expander has a property that varies alongan axial length of the expander, the property selected from the groupconsisting of outer diameter, physical form, composition andcombinations thereof, for the purpose of providing a gradient ofautofrettaging along a length of the structure.
 11. A process forautofrettaging wherein the process comprises imposing plasticdeformation on an inner surface of a structure and elastic deformationon a more outer portion of the structure, the process comprising:providing a structure having an outer surface and an inner surface;providing an expander that occupies a majority of the interior volume ofthe structure; placing the expander adjacent the inner surface of thestructure; and applying a compressive force onto the expander whereinthe compressive force induces predominantly elastic strain to the outersurface of the structure and elastic plus plastic strain to the innersurface of the structure further comprising placing a liner intermediatebetween the expander and the structure.
 12. The process of claim 11,wherein the compressive force induces predominantly elastic strain tothe outer surface of the structure and elastic plus plastic deformationto the liner within the structure.
 13. The process of claim 11, whereinthe compressive force causes bonding between the outer surface of theliner and the inner surface of the structure.
 14. A process forautofrettaging wherein the process comprises imposing plastic strain onan inner surface of a hollow cylindrically shaped structure and elasticdeformation on a more outer portion of the structure, the processcomprising: providing a hollow cylindrically shaped structure having anouter surface and an inner surface; providing an elastomer expander, theelastomer expander dimensioned and shaped to be inserted within thehollow cylindrically shaped structure and occupy a majority of thevolume of the hollow cylindrically shaped structure; inserting theelastomer expander within the hollow cylindrically shaped structure; andapplying an axial compressive force onto opposite ends of the elastomerexpander, the axial compressive force inducing predominantly elasticstrain on the outer surface of the hollow cylindrically shaped structureand elastic plus plastic strain on the inner surface of the hollowcylindrically shaped structure.
 15. The process of claim 14, wherein theelastomer expander has a Poisson's ratio between 0.3 and 0.5.
 16. Theprocess of claim 14, wherein the hollow cylindrically shaped structureis a gun barrel.
 17. The process of claim 14, wherein a magnitude of theaxial compressive force is a function of where the elastomer expander islocated within the hollow cylindrically shaped structure, for thepurpose of providing a gradient of autofrettaging along a length of thehollow cylindrically shaped structure.
 18. The process of claim 14,wherein the elastomer expander has a property that varies along an axiallength of the elastomer expander, the property selected from the groupconsisting of outer diameter, physical form, composition andcombinations thereof, for the purpose of providing a gradient ofautofrettaging along a length of the hollow cylindrically shapedstructure.
 19. The process of claim 14, further comprising placing aliner intermediate between the expander and the hollow cylindricallyshaped structure.
 20. The process of claim 19, wherein the axialcompressive force induces predominantly elastic strain to the outersurface of the hollow cylindrically shaped structure and elastic plusplastic strain to the liner within the hollow cylindrically shapedstructure.
 21. A process for imposing plastic deformation on an innersurface of a metal structure, the process comprising: providing a metalstructure having an outer surface and an inner surface; providing anexpander; placing the expander adjacent the inner surface of the metalstructure; and applying a compressive force onto the expander wherein amagnitude of the compressive force is a function of where the expanderis.
 22. The process of claim 21, wherein the expander comprises amaterial that has a form selected from the group consisting of a solidform, a particulate and combinations thereof.
 23. The process of claim22, wherein the expander comprises a material that is a solid elastomeror a solid plastic.
 24. The process of claim 22, wherein the expandercomprises elastomeric particles or plastic particles or a combinationthereof.
 25. The process of claim 22, wherein comprises a material thathas a Poisson's ratio between 0.4 and 0.5.
 26. The process of claim 22,further comprising placing a liner intermediate between the expander andthe hollow cylindrically shaped structure.